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Postretirement Benefit Obligations - Narrative (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Sep. 30, 2018
Retirement Benefits [Abstract]        
Pension contributions $ 3.3 $ 6.6 $ 2.0 $ 12.0